I didn't want my first posts on this forum to be about my technical problems, but here it goes.
Long story short, I start my car few weeks ago, got the yellow check engine light, accompanied by the lack of power indicator. Got a little scared ( this happened about 2 days after I changed my oil, filters etc ) so I ran to the dealership. They diagnosed it, and told me it had to do with "Valvetronic internal fault", they deleted the errors, did not appear again, all fine. Drove back home happy it was something minor. After a few days, the dashboard lighted up with the same 2 indicators, same routine, back to the dealership, they did a diagnose and told me they will add a new software and reset the sensors. Done deal, car did not show any errors, all fine. After a few days, boom, the lights were back on. Happily, I went to the dealership to see what the heck was disturbing my peace. They told me that since neither technique worked, I had to replace the Valvetronic Actuator, the Eccentric shaft sensor along with few seals. I was given the price of about 1200$ for the operation. I am wondering if this will solve my problems. The car is going into limp mode when I start it but after about 1km goes back to full power, rough idling is also a symptom ( car idles around 500 - 900 rpm, given the fact it should stay at around 850rpm ). The error codes were 2A67 and 2A6B, both regarding the Valvetronic. The parts will arrive on monday, I will keep you posted in the meantime. I kept on studying about this and learned it's a common fault for the E90's. Is there something else causing them to malfunction?

Do a search on "eccentric shaft". There's a huge thread about it floating around. There's a seal that fails and let's oil into the sensor which screws it up causing those valvetronic error codes. The op of that thread had almost the same issues you had. Eventually the car will have almost zero power. I have no experience with this myself but others in this section have.